Sunday, 11 November 2012

IR filters

When taking pictures, we normally want the resulting image to accurately depict the reality. However, there are times when we rather want to depict a scene in a way that is not consistent with the reality as we see it.

This may sound strange. As the camera industry has spent an enormous effort to realistically depict the scene, why would we want to distort it?

However, keep in mind that one of the current crazes within popular photography is instagram, hipstamatic, and so on, which is all about distorting the images, and making them look unnatural.

Another way in which photos can be distorted is selective focus. The combination of a large sensor and a large aperture can give you an out of focus background when taking a portrait photo, for example. The rendering of out of focus regions is called bokeh, and the effect can be much more pronounced than what you would see with your own eye. Hence, one could argue that this is a distortion of the reality.

Another type of distortion is of course to use a lens with an unrealistic projection, for example a fisheye lens.

In this article, I am looking at another type of distortion: To use an IR filter to keep off the visible light, and hence, to photograph light outside of the visible spectrum.

A number of different types of IR filters are available. Usually, they are categorized by the cut-off frequency. Visible light roughly goes from 340nm to 740nm, with ultra violet (UV) to the left of the visible spectrum, and infrared (IR) to the right, see the illustration below.

The idea of the IR filter is to remove light which is to the left of the cut-off frequency, only leaving the infrared parts of the spectrum.

I tried two filters, both where bought online at a low price:

The filter to the left is labelled with "IR950", meaning that it cuts off all light to the left of 950nm. Hence, it blocks all visible light completely, and even some of the infrared light. The filter to the right is less extreme, labelled "IR680": It retains some visible red light, but cuts off the larger part of the visible spectrum. I start with the one rated the lowest:

IR680

This filter looks completely black, however, when holding it up towards the sun, you can see that it lets some red light through:

To see how it affects the images, I have taken the same image without and with the filter. Here they are:

Based on the exposure settings, we see that the IR680 filter steals about eight stops of exposure, which is a lot. Normally, you need to use a tripod and a slow shutter speed when using an IR filter.

So how is this interesting? It is just a very red tinted photography. However, when opening the RAW image, one can adjust the white balance, which gives a result like this, leaving some false colours:

Another possibility is to desaturate the images, to create black/white photos. Here are the two images desaturated:

Without filter

With IR680

The desaturated images are not that different.

Here is another example image, taken with the IR680 filter, and no desaturation in post processing:

IR950

This filter looks even blacker than the IR680 filter. When holding it towards the sun, it still looks completely black. I tried it on the GF3, GH1 and GH2 cameras, and they could all just see black when using the filter. So my conclusion is that this filter is too extreme to be used with most digital cameras.

Different sensors can have different types of filters in front of the photosites, and hence, react differently to infrared light. That is why I tested three cameras before giving up. It is well known that some cameras are better than others for infrared photography.

I also tested two different lenses, as I have heard that the coating on some lenses can block infrared light.

IR850

I also tried a third IR filter, with a cut-off frequency of 850nm, not pictured above. This filter looks completely black to the human eye, but the camera can see a little bit through it.

I tested it with the Lumix G 14mm f/2.5 lens. Here are pictures of the same scene without and with the filter:

Without filter, ISO 160, 1/4000s, f/2.5

With IR850, ISO 1250, 1/3.2s, f/2.5

The IR850 filter appears to remove too much light for the Panasonic GH2 camera sensor, and is not very useful.

Note that there is a lot of flare in the image taken with the filter. This is not due to the infrared light, but rather due to the strong light source in the centre of the image, and the poor quality of the filter glass. I have written about this effect of using filters here.

Conclusion

I think the IR680 filter lets through too much visible light, and hence, it doesn't create that special effects. On the other hand, the IR950 filter is too extreme. It removes all the light that the camera can see, and hence, is useless.

I would guess that something in between would give more interesting results. Perhaps a filter with a cut-off at around 720nm would be better. Such a filter removes virtually all visible light, and allows you to photograph a band of infrared light, given that the camera sensor is sensitive enough in this spectral range.

I've sucessfully recored test shots hand held with a 950nm filter using a f/1.4 lens on my G1, though the 720nm ones work better.

Your 680nm shot doesn't show the typical features of IR photography (bright foliage & contrasty skies) IMO filters letting through visible wavelengths are only really practical if you have a camera with the hot mirror removed (making the camera much more sensitive to IR).

Desaturated images taken in true IR will usually be very different in character to B&W images. Infra red effects skin tones (less blemishes are visible), sees through haze better, gives bright foliage, and can even see through some plastics that are opaque to visible light.

Your shot with the 850nm filter shows a significant hot spot, which is a problem encountered with many lenses. It is caused by reflections of IR within the lens due to coatings which are not designed for infra red. It's unlikely the filter itself has a significant impact on it. If you check infra red forums or groups on flickr etc you'll often find long lists of lenses that are prone to hotspots (or don't). My 14-42 unfortunately is rather prone to them.